1. Field of the Invention
The present invention relates to a projection system for the display of aerial images and more particularly, to a projection system for displaying three-dimensional video images having a low-cost plastic spherical mirror.
2. Description of Related Art
Aerial image projection systems are known in the art. Such systems utilize a plurality of optical elements such as mirrors, Fresnel lens and optical filters or polarizers to project an image of an object into space. The optical elements and the object are positioned in a housing to define an optical path. Depending on the arrangement and selection of the optical elements, the projected image is visible either within the dimensions of the housing or a short distance in front of the housing. Examples of aerial projection systems include U.S. Pat. No. 5,311,357, issued May 10, 1994, U.S. Pat. No. 5,552,934, issued Sep. 3, 1996, U.S. Pat. No. 4,802,750, issued Feb. 7, 1989, and U.S. Pat. Des. No. 435,043 issued Dec. 12, 2000.
Prior art aerial projection systems are expensive because of the cost of optical components required to project the aerial image of an object. More specifically, such systems use one or more concave glass mirrors in the optical path together with one or more glass polarizers maintained in a fixed orientation with respect to a stage where an image is positioned. Unfortunately, 15-inch concave glass mirror or spherical mirror costs well over $1,000 and polarizers cost about $850. Clearly, glass spherical mirrors and polarizers are major contributors to the high cost of the prior art aerial projection system. Not only expensive, these glass spherical mirrors and polarizers are also very heavy so adequate support must be provided. Accordingly, a heavy box-like housing is used to maintain the orientation of the optics with respect to the object.
As mentioned above, glass has been the conventional material of choice for use as the spherical mirror. One of the most important reasons is because plastics technologies were not as developed as they are today. In other words, the tools and materials were not available as they are today. The metal mold tolerances and the resulting parts can be specified and held in the tens of thousandths of an inch. Materials used today are more sophisticated; the plastics are able to emulate the thermal stability and durability similar to that of glass, and to endure the type of operating conditions in the past that only glass could have tolerated.
Furthermore, the glass spherical mirrors are expensive because of the secondary operations needed to prepare the mirror surface after it is heat formed or slumped to shape. These secondary operations include annealing, grinding and polishing. The annealing process is used to strengthen the glass so that it is strong enough to undergo the grinding and polishing operation, as well as adding the additional strength needed to resist breakage during usage. The grinding and polishing stages are necessary because of the limits of the tolerance capabilities of glass forming molds and the physical nature of glass. Unfortunately, the grinding and polishing stages require a considerable amount of manual processing for producing a finished product; therefore, they are often considered semi-automated processes.
In addition, glass also has the serious drawbacks of breakage, weight, and expensive shipping costs. To try to overcome the limitations and drawbacks of glass, a low-cost glass forming process was developed. However, the low-cost glass forming process did not provide an acceptable surface finish, and the resulting cost reductions were not comparable to that of plastic. Clearly, what is needed is a method and system for manufacturing a plastic part to reduce the weight of a spherical mirror to approximately one-third that of glass, and for making a low-cost plastic spherical mirror of comparable performance to glass spherical mirror. As a result, an aerial projection system that is lightweight, inexpensive and easily transported from one location to another can then be realized.
While prior art aerial projection systems generate visually captivating aerial images, there are a number of problems that limit use of aerial projection systems in a wide variety of applications. Accordingly, prior art aerial projection systems are typically used in museums or retail stores to display expensive items where the object being displayed can be kept safely out of the reach of the observer.
Prior art aerial projection systems typically use a three-dimensional object as the source of the image. For example, a small statue may be placed on a pedestal and brightly lighted with spotlights. The three-dimensional image of the statue is projected through a display window and viewed by observers who are positioned in front of the display window as if it were floating in air.
One problem with using an object as the source of the projected image is the difficulty and expense associated with changing the image. Thus, to maintain the viewer's interest and to preserve the novelty of the projected image, the object must be constantly changed. This is a labor-intensive process as an attendant must open a door in the housing, remove the object, position a new object and verify that it is properly positioned on the display pedestal.
To overcome this limitation, aerial display systems have attempted to utilize a video display device instead of a physical object as the image source. Unfortunately, the video images appear together with an image of the display screen. Thus, rather than displaying a floating image, the aerial image appears to the observer as a floating video display screen thereby rendering the illusion of an image floating in air ineffective. What is needed is an aerial projection system capable of displaying video images without the video display screen being visible to the observer.
Another problem associated with the display of video images arises from the display device itself. Specifically, video monitors use a flat piece of optical quality glass behind which the image is generated. This glass tends to reflect external images that pass through the optics in the optical path. The reflected image is viewable by the observer resulting in a noticeable double aerial image. Clearly, what is needed is an aerial projection system that eliminates reflected images from the displayed aerial image.
Yet another problem with prior art display of video images arises when the object of attention moves off screen. More specifically, when an image transgresses beyond a boundary of the display, the observer immediately detects the edge condition and the illusion of a floating image is lost. Accordingly, what is needed is a method for displaying an aerial image in a manner that does not suggest that the image is generated by a video display.
Thus, a better system and method for projecting aerial images is needed. More specifically, what is needed is an aerial projection system for projecting images at video rates that is lightweight and inexpensive.